This invention relates to recovering particles from a fluid-particulate suspension in a cyclone separator, and subsequently stripping entrained fluid from the particles.
Cyclonic separation involves separating a mixture of two or more phases, such as for example, suspensions of particulates in a carrier fluid, under a centrifugal force generated by centripetal motion. A cyclone separator, or cyclone, is a mechanical device to perform centrifugal separation of flowing mixed phases. Uses of cyclonic separation methods can include unit operations to purify a phase, to concentrate a phase, to terminate chemical and physical interactions between mixed phases, or combinations thereof.
Cyclone separation is common in fluid catalytic cracking (FCC) technology where hydrocarbon vapors and particulate catalysts come into intimate contact. FCC processes, which employ catalyst fluidization and hydrocarbon atomization for conversion reactions, require rapid mixing and separation of fluid and solid phases to maintain control over product yields. Developments in cyclonic separation technology have driven FCC technology toward increased catalyst activities; conversely, increases in catalyst activity spur the need for development of higher cyclone efficiencies.
Achieving high productivity from FCC systems requires methods for the regulation of contact times between the catalyst and the hydrocarbons. Controlling the contact times depends increasingly on rapid cyclonic separation, as contact time is the key to optimizing process yields. FCC systems are designed to operate using typical interphase contact times between 0.2 and 10 seconds, desirably between 1 and 4 seconds.
In any cyclonic separation of a suspension, some residual carrier fluid will remain entrained with and adsorbed onto the particles, even after the particles have separated and settled out of the carrier fluid. Accordingly, because of high reaction rates in FCC applications, another important consideration of cyclonic separation is the displacement of the residual carrier fluid from the disentrained catalyst particles. This displacement will stop reactions between the catalyst and residual hydrocarbon fluids, helping to control conversion product profiles and to minimize “delta coking” on the catalyst.
One method of displacing residual fluid from disentrained catalyst particles includes the introduction of a stripping gas, such as for example, air, steam, ammonia, flue gas, or similar gases, to diffuse the residual hydrocarbons away from the disentrained catalyst particles.
U.S. Pat. No. 3,802,570 to Dehne discloses a cyclonic separation method to stabilize a vortex in a cyclone separator for improved phase separation through reduced re-entrainment of solids.
U.S. Pat. Nos. 4,455,220 and 4,692,311 to Parker et al. disclose injecting air and ammonia via utility piping into an annulus below the cyclone, and diffusing the gases through a sintered, annular, stainless steel ring into the catalyst bed beneath a vortex stabilizer in the cyclone. The gases are injected at a rate of about 1 to 4 grams of gas per kilogram of catalyst separated in the cyclone.
U.S. Pat. Nos. 4,502,947 and 4,741,883 to Haddad et al. disclose a method of closing a pathway of FCC-cracked hydrocarbon vapors wherein the catalyst suspended in the vapors exits the FCC riser-reactor and is conducted via an unsealed plenum through a succession of staged cyclone separators in series, wherein the cyclones are mounted in the FCC catalyst-stripping vessel. Stripping gas filling the stripping vessel blends with the suspension through an unsealed annular junction in the plenum, downstream of a first-stage riser cyclone.
U.S. Pat. No. 4,778,488 to Koers discloses a cyclone separator for removing hot particles of shale, tar sand, or coal from a gas-borne suspension in a pyrolytic retorting process. A pipe manifold is inserted into the bottom section of the cyclone for introducing stripping gas into the dense bed of separated solids.
U.S. Pat. No. 5,569,435 to Fusco et al. discloses an open-bottomed, diplegless, open-topped cyclone design for receiving a flow of suspended solids from the FCC riser. The design accommodates unsteady state FCC riser conditions, and is said to provide efficient separation of solids. Stripping gas is introduced into an upper section of an FCC catalyst-stripping vessel housing the cyclone. Up to 20 percent of the stripping gas enters the cyclone through the cyclone's open bottom, and the balance of the stripping gas enters an annular opening in the cyclone duct which discharges vapors from the top of the cyclone.
U.S. Pat. No. 5,869,008 to Dewitz discloses an open primary cyclone with the lower end of the cyclone inserted into the catalyst bed of the FCC stripping vessel enclosure. A piping manifold is installed within the open cyclone, beneath the surface of the cyclone's catalyst bed, for injecting stripping gas into the bed.